1. Field of the Invention
The present invention relates to a drive control device for an ultrasonic motor, and more particularly relates to a drive control device for an ultrasonic motor, which can perform control so as to ensure that the drive frequency lies within a predetermined resonance frequency band.
2. Description of Related Art
Because an ultrasonic motor provides high output power at low speed, and for other reasons, it has very suitable characteristics for use in actuators etc., and accordingly it has much potential in various fields of industrial application.
The general construction and function of a rotating type ultrasonic motor will now be explained by way of example. FIG. 14 is a sectional view showing the general construction of a typical such rotating type ultrasonic motor. Referring to this figure, the reference numeral 11 denotes the movable member of the ultrasonic motor, in other words the rotor thereof in the case of a rotating type ultrasonic motor, and to this rotor 11 there is fixed a slider member 11a. The reference numeral 12 denotes a stator of the ultrasonic motor, which comprises an elastic member 12a and a piezoelectric transducer element 12b fixed to said elastic member 12a. The piezoelectric transducer element 12b converts electrical energy into mechanical energy, thus playing the role of an electrical energy to mechanical energy conversion element. The stator 12 is held fixed in place between a tubular support member 13a, which is formed with an internal step and a screw thread leading to said internal step, and a securing ring 13b formed with an external screw thread, which is screwed down into said tubular support member 13a so as to clamp the stator 12 against said internal step thereof. Further, the rotor 11 is pressed against some element not shown in the figure which exerts resistance to said pressure, whereby the rotor 11 is pressed against the stator 12 into intimate contact therewith.
FIG. 15 is a figure showing a typical arrangement for electrodes formed on the piezoelectric transducer element 12b in the rotating type ultrasonic motor of FIG. 14. These electrodes divide up the piezoelectric transducer element 12b into a plurality of electrode portions. These electrodes transmit a supply of electrical power for driving the ultrasonic motor, and also serve for sensing the operational state of the ultrasonic motor. Those of the electrodes designated in FIG. 15 by circular black dots are electrodes of positive polarity, while the electrodes in FIG. 15 which are not designated by any circular black dots are electrodes of negative polarity. The electrode groups 14A and 14B are input electrode groups as shown in the figure, and each electrode in said electrode groups 14A and 14B has circumferential extent one half of a wavelength .lambda., and neighboring electrodes in these electrode groups 14A and 14B are at a distance apart which yields a phase difference .lambda./4. An electrode 14M of circumferential extent .lambda./4 is provided in one of the gaps defined between the two electrode groups 14A and 14B, and is normally used as a detector electrode for detecting the vibrational condition of the progressive wave which is set up in the elastic member 12a. Another electrode 14C of circumferential extent 3*.lambda./4, generally termed a common electrode, is provided in the other one of the gaps defined between the two electrode groups 14A and 14B, and is normally used as a ground electrode. The piezoelectric transducer element 12b is fixed to the elastic member 12a, and AC drive signals differing in phase by .pi./2 are supplied to the two input electrode groups 14A and 14B, whereby a progressive wave is generated in the elastic member 12a and the rotor 11 which is pressed against said elastic member 12a is turned by this progressive wave.
In order to drive such an ultrasonic motor stably and reliably, the frequency of the AC drive signals which are supplied to the two input electrode groups 14A and 14B (hereinafter referred to as the drive frequency) is required to be set to the most suitable frequency for the individual ultrasonic motor in the particular conditions of use at the moment. This most suitable drive frequency is a frequency which is somewhat higher than an inherent resonance frequency of the ultrasonic motor, and, when the ultrasonic motor is driven at this most suitable drive frequency, the amplitude of the progressive wave is large due to resonance of the elastic member 12a, whereby the efficiency of the ultrasonic motor is at a desirably high level. If the drive frequency for the ultrasonic motor deviates from the most suitable frequency in the neighborhood of this resonance frequency, then the amplitude of the progressive wave becomes smaller and said progressive wave may become unable to drive the ultrasonic motor, or unusual sounds may be generated during motor operation, or it may become impossible to drive the ultrasonic motor properly and it may start to rotate in the opposite direction from the desire operational direction.
However, the resonance frequency of the ultrasonic motor is always changing due to change of temperature and change of load, and therefore it is necessary to change the setting for the most suitable drive frequency for the ultrasonic motor according to this changed resonance frequency. Although many and various drive control devices for an ultrasonic motor have been proposed in which an attempt is made always to provide the most suitable drive frequency even if the resonance frequency changes, the following types of such drive control devices are representative:
(1) In a first type of drive control device, the output voltage from the detector electrode is detected, and the drive frequency is controlled so as to bring this output voltage to a voltage value corresponding to the most suitable drive frequency.
(2) In a second type of drive control device, the phase difference between the waveform of the output voltage from the detector electrode and the waveform of an input voltage supplied to an input electrode group is detected, and the drive frequency is controlled so as to bring this phase difference to a desire phase difference value.
(3) In a third type of drive control device, the phase difference between the waveform of an input voltage supplied to an input electrode group and the waveform of the drive current flowing through said input electrode group is detected, and the drive frequency is controlled so as to bring this phase difference to a predetermined phase difference value.
Although various other such drive control devices for an ultrasonic motor have been proposed, explanation thereof will be curtailed in the interests of brevity.
FIG. 16 is a block diagram showing the structure of a prior art type drive control device for an ultrasonic motor such as the one shown in FIGS. 14 and 15. This drive control device is an example of one which has been proposed by the applicant of the present application, and is of the type described generally in (2) above. If particular details are required, reference should be made to Japanese Patent Laying-Open Publication 62-251490. This drive control device for an ultrasonic motor comprises: an oscillator 21 which outputs the drive frequency f; a waveform shaping device 22; a phase shifter 23 which shifts the drive signal input to the input electrode groups 14A and 14B by exactly .pi./2; power amplifiers 24 and 25; a phase difference calculation circuit 29 which calculates the phase difference .PHI. between the output voltage waveform output from the detector electrode 14M and the drive voltage waveform input to the input electrode group 14A; a comparator 30 which compares this phase difference .PHI. with a reference phase difference .PHI.opt and outputs the amount .DELTA..PHI. of deviation between them; a referencer 31 which outputs the reference phase difference .PHI.opt; and a .PHI. to f conversion calculator 32 which converts this amount .DELTA..PHI.of deviation into an amount .DELTA..function. by which the drive frequency f should be increased or decreased. Further, the reference numerals 27 and 28 denote load matching inductors.
The phase difference .PHI. between the output voltage waveform output from the detector electrode 14M and the drive voltage waveform input to the input electrode group 14A is calculated by the phase difference calculation circuit 29. The comparator 30 compares this phase difference .PHI. with a reference phase difference .PHI.opt which is set by the referencer 31, and outputs the amount .DELTA..PHI. of deviation between them. Now, this reference phase difference .PHI.opt is the phase difference when the ultrasonic motor is being stably driven by the most suitable drive frequency, which is slightly higher than its resonance frequency. Next, the .PHI. to f conversion calculator 32 converts this amount .DELTA..PHI. of deviation into an amount .DELTA..function. by which the drive frequency f should be increased or decreased, and this amount .DELTA..function. is output to the oscillator 21. The oscillator 21 then increases or decreases the drive frequency f by just this amount .DELTA..function.. In this manner, the drive frequency f for the ultrasonic motor is always controlled to be the most suitable drive frequency which is somewhat higher than the resonance frequency of the ultrasonic motor, so that said ultrasonic motor can thereby be stably driven.
FIG. 17 shows together, the way in which the drive speed N of the ultrasonic motor varies with respect to the drive frequency f, and the way in which the phase difference .PHI. between the drive voltage waveform and the detector voltage waveform varies with respect to the drive frequency f. In this figure, it is shown that initially the ultrasonic motor was being driven so as to operate according to the characteristic expressed by the drive curve NA and the phase difference curve .PHI.A, and that, due to change of temperature or the load on the ultrasonic motor or the like, the driving conditions of the ultrasonic motor altered so that its operation came to be according to the characteristic expressed by the drive curve NB and the phase difference curve .PHI.B. In this figure, whichever of these characteristics properly describes the operation of the ultrasonic motor, the phase difference .PHI. is shown as being the same value for the most suitable drive frequency f1 or f2 for the ultrasonic motor, and therefore, if this phase difference value is set to the reference phase difference value .PHI.opt described above, the ultrasonic motor can thereby always be stably driven.
A resonance frequency of the ultrasonic motor exists for each order of resonance, and normally among them the resonance frequency band of the order that shows the best drive characteristic is set as the drive frequency band for the ultrasonic motor, and the ultrasonic motor is drive controlled so that its driving frequency is kept within that drive frequency band. Hereinafter in this specification, this resonance frequency band of the order that shows the best drive characteristic for the ultrasonic motor will be termed the most suitable resonance frequency band.
On the other hand, if for example the ultrasonic motor is being driven at extremely low rotational speed, the variation in speed is small in the case of driving said ultrasonic motor with a frequency of a resonance frequency band other than said most suitable frequency band, rather than driving said ultrasonic motor with a frequency in the most suitable resonance frequency band, and such operation exhibits excellent characteristics. Accordingly, when the ultrasonic motor is to be driven at normal speed, said ultrasonic motor is driven with a frequency in the most suitable resonance frequency band, and, when said ultrasonic motor is to be driven at extremely low rotational speed, the drive frequency for driving said ultrasonic motor is switched over from the most suitable resonance frequency band to another resonance frequency band, and thereafter said ultrasonic motor is driven with a frequency in said other resonance frequency band.
However, with such a prior art type of drive control device for an ultrasonic motor, when due to temperature variation or change of the load on the motor the drive frequency deviates from the most suitable resonance frequency band or from the resonance frequency band for operation at extremely low rotational speed, then problems can arise of inconveniences and improper operation, such as the drive frequency not reliably returning to said most suitable resonance frequency band or to said resonance frequency band for operation at extremely low rotational speed, or of the driving operation of the ultrasonic motor becoming unstable, or of the generation of unusual noise or the like.
Now, the resonance frequency band which is used for the drive frequency band for the ultrasonic motor will be explained. Considering the piezoelectric transducer element 12b shown in FIG. 15 as an example, because the circumferential extent of any one of the electrodes included in the input electrode groups 14A and 14B is .lambda./2, therefore the total circumferential extent of the input electrode group 14A is equal to 5*.lambda., as is the total circumferential extent of the input electrode group 14B. Further, the circumferential extent of the common electrode 14C is 3*.lambda./4, and the circumferential extent of the detector electrode 14M is .lambda./4, so adding these all together the total circumferential extent of all the electrodes is 11*.lambda.. It is usual for the number of waves in the progressive wave generated on the stator 12 when the ultrasonic motor is being driven to agree with the number of waves on the divided electrode of the piezoelectric transducer element 12b. I.e., with the piezoelectric transducer element 12b of circumferential extent 11*.lambda. shown in FIG. 15, the eleventh order resonance frequency band which generates the eleventh order progressive vibration wave is usual; and this is a most suitable resonance frequency band for use as a drive frequency band for the ultrasonic motor.
As shown in FIGS. 18 and 19, on the low frequency side of this eleventh order resonance frequency band there exist in order lower order resonance frequency bands, i.e. the tenth order resonance frequency band, the ninth order resonance frequency band, etc., while on the high frequency side of this eleventh order resonance frequency band there exist in order higher order resonance frequency bands, i.e. the twelfth order resonance frequency band, the thirteenth order resonance frequency band, etc..
FIG. 18 is a figure showing, together, the way in which the drive speed N of the ultrasonic motor varies with respect to the drive frequency F, and the way in which the output voltage VM from the detector electrode varies with respect to the drive frequency F. With a drive control device of the type described generally in (1) above, the drive frequency is set to F11 so as to make the output voltage from the detector electrode be equal to VM1. This drive frequency F11 is the frequency of the most suitable eleventh order resonance frequency band. Further, when the ultrasonic motor is to be driven at extremely low rotational speed, the drive frequency is set to F120 so as to make the output voltage from the detector electrode be equal to VM2. This drive frequency F120 has more excellent drive characteristics when driving the ultrasonic motor at extremely low speed than does the frequency of the most suitable eleventh order resonance frequency band, and for example may be the frequency of the twelfth order resonance frequency band.
However, there are other frequencies F9, F10, F12, F13 . . . which make the output voltage VM from the detector electrode to be equal to VM1, as well as the frequency of the most suitable eleventh order resonance frequency band, and similarly there are other frequencies F90, F100, F110, F130 . . . which make the output voltage VM from the detector electrode to be equal to VM2, as well as the frequency of the twelfth order resonance frequency band. If temporarily the drive frequency F should wander from the previously set most suitable eleventh order or twelfth order resonance frequency band, whichever of them it is set to, and should come to be equal to one of these other frequencies, then it will not return to the proper frequency band.
FIG. 19 is a figure showing, together, the way in which the drive speed N varies with respect to the drive frequency F, and the way in which the phase difference .PHI. between the detector voltage and the drive voltage varies with respect to the drive frequency F. With a drive control device of the type described generally in (2) above, the drive frequency F for the ultrasonic motor is set so as to make the phase difference .PHI. between the detector voltage and the drive voltage to be equal to a phase difference value .PHI.1 corresponding to the drive frequency F11' of the most suitable eleventh order resonance frequency band. Further, when the ultrasonic motor is to be driven at extremely low rotational speed, the drive frequency F is set so as to make the phase difference .PHI. to be equal to .PHI.2. This drive frequency F120 has more excellent drive characteristics when driving the ultrasonic motor at extremely low speed than does the frequency of the most suitable eleventh order resonance frequency band, and for example may be the phase difference .PHI.2 corresponding to the frequency of the twelfth order resonance frequency band.
However, there are other frequencies F10', F12', F13' . . . which make the phase difference .PHI. to be equal to .PHI.1, as well as the frequency of the most suitable eleventh order resonance frequency band, and similarly there are other frequencies F130' . . . which make the phase difference .PHI. to be equal to .PHI.2, as well as the frequency of the twelfth order resonance frequency band. If temporarily the drive frequency F should wander from the previously set most suitable eleventh order or twelfth order resonance frequency band, whichever of them it is set to, and should come to be equal to one of these other frequencies, then it will not return to the proper frequency band.
A detailed explanation will not be given in this specification in view of the desirability of brevity, but the same type of problem arises with a drive control device of the type described generally in (3) above, in which the drive frequency is controlled based upon the phase difference between the waveform of an input voltage supplied to an input electrode and the waveform of the drive current flowing through said input electrode.
It has been conceived of to control the drive frequency for the ultrasonic motor to be the frequency of the most suitable resonance frequency band or the frequency of a resonance frequency band for driving the ultrasonic motor at extremely low rotational speed, and, in order to drive the ultrasonic motor in a stable fashion, to limit the output frequency band of the setting circuit for setting the drive frequency, or the output frequency band of the drive frequency generating device, to the most suitable resonance frequency band or to the frequency of the resonance frequency band for driving the ultrasonic motor at extremely low rotational speed, whereby it may be ensured that the drive frequency for the ultrasonic motor is never included in any other resonance frequency band. However this approach is not particularly desirable, because the construction in order to provide this function of the drive frequency setting circuit or the construction of the drive frequency generating circuit becomes complicated, and further circuitry is required in order to compensate for changes in the output frequency due to the effects of changes in temperature and of changes in the load on the ultrasonic motor and the like.
Further, since the resonance frequency band of the ultrasonic motor varies due to changes in temperature and changes in load, there are problems in making the frequency band of the drive frequency setting circuit or the frequency band of the drive frequency generating circuit always agree with the most suitable resonance frequency band or the resonance frequency band for driving the ultrasonic motor at extremely low rotational speed.
Yet further, since individual ultrasonic motors, even of the same specification, inevitably differ from one another with regard to resonance frequency band, it is necessary to perform individual adjustment for each individual ultrasonic motor in order to set the frequency band of its drive frequency setting circuit or the frequency band of its drive frequency generating circuit appropriately, and a problem thereby arises of great labor in manufacture of the ultrasonic motor, which inevitably entails high cost.
In order to prevent deviation of the drive frequency for the ultrasonic motor by limiting the output frequency band of the drive frequency setting circuit or of the drive frequency generating circuit, it is necessary to set said output frequency band narrower than the most suitable resonance frequency band or the resonance frequency band for driving the ultrasonic motor at extremely low rotational speed, so as to take into account deviation of the resonance frequency band of the ultrasonic motor due to changes in temperature and changes in load, as described above. However, when the output frequency band of the drive frequency setting circuit or of the drive frequency generating circuit is set narrower than the most suitable resonance frequency band or than the resonance frequency band for driving the ultrasonic motor at extremely low rotational speed, then it is not possible to set the drive frequency for the ultrasonic motor in the neighborhood of the upper limit value or in the neighborhood of the lower limit value of the most suitable resonance frequency band or of the resonance frequency band for driving the ultrasonic motor at extremely low rotational speed, and accordingly the range of variation of speed of the ultrasonic motor is undesirably narrowed.
Although by way of example the above explanation of the problems associated with the prior art has been made in terms of an ultrasonic motor which comprises the piezoelectric transducer element 12b shown in FIG. 15, similar problems arise with other prior art types of ultrasonic motor of different construction.